1.2.1 Systems software

Question 1

what is an operating system?

Answer 1

a collection of programs that work
together to provide an interface between the user and computer

Question 2

what are the functions of an operating system?

Answer 2

• memory management
• resource management
• input/output management
• interrupt management
• security
• user interface
• utility software

Question 3

what is paging?

Answer 3

when main memory is split physically into equal sections

Question 4

what is segmentation?

Answer 4

when main memory is split logically into variable sizes

Question 5

what are similarities between paging and segmentation?

Answer 5

-both used when RAM space is insufficient
-can cause disk thrashing

Question 6

what is virtual memory?

Answer 6

a memory management technique that allows a computer to use a portion of it’s hard drive as if it were RAM

Question 7

what is virtual memory used for?

Answer 7

-temporarily moves sections of programs not currently being used
-frees up memory for other programs in RAM

Question 8

what is an interrupt?

Answer 8

an instruction that has high priority so it stops the current FDE cycle from being completed.

Question 9

what are the stages of the interrupt
service routine (ISR)?

Answer 9

-interrupt register checked at the end of each FDE cycle
-if interrupt has a higher priority to the current instruction, the current contents of the registers in the CPU are transferred into a stack
-interrupt is completed using the FDE cycle
-interrupt register checked again
-if interrupts are of a higher priority, they will be put at the top of the stack to be completed first
-if there are no interrupts with a higher priority, the contents of the stack are popped and place back into the special memory registers
-the FDE cycle renews as before

Question 10

what are the types of operating system?

Answer 10

-distributed
-real time
-multi-tasking
-multi-user
-embedded

Question 11

what is a distributed operating system?

Answer 11

coordinates communication between many individual computers but it appears to be one system to the end user

Question 12

what is an embedded operating system?

Answer 12

used in computers that serve a specific purpose
(washing machines)

Question 13

what is a multi-tasking operating system?

Answer 13

manages the computer’s resources to provide the illusion to the user that it is possible to switch between tasks
(laptops- working on a document while listening to music)
(a single core computer can only run one application at a time, but the OS can switch between tasks quickly to make it look like multiple tasks are running simultaneously)

Question 14

what is a multi-user operating system?

Answer 14

provides the facilities for multiple users to access the same system
(supercomputers)

Question 15

what is a real-time operating system?

Answer 15

processes data within a guaranteed amount of time
(medical devices- heart rate monitors)

Question 16

what are the functions of the BIOS (Basic Input Output System)?

Answer 16

-POST (Power On Self Test) which ensures that all hardware is correctly connected and functional
-checking the CPU clock, memory and processor
-testing for external memory devices

Question 17

what is the use of a device driver?

Answer 17

enables the operating system to interact with hardware devices

Question 18

what is a virtual machine?

Answer 18

a software-based computer that acts like a physical computer, but is only a digital representation of one

Question 19

why are virtual machines used?

Answer 19

-testing programs
-protection from malware
-running software compatible with different versions and types of operating systems

Question 20

what is scheduling?

Answer 20

allocating time slots for instructions to be completed

Question 21

what are the types of scheduling?

Answer 21

-first come first served
-round robin
-shortest remaining time
-shortest job first
-multilevel feedback queue

Question 22

what are the benefits of scheduling?

Answer 22

-increases efficiency
-reduces bottle neck
(instructions completed one at a time)

Question 23

what is first come first served?

Answer 23

processes are completed in the order which the processor receives them
-simplest method to implement
-long wait times

Question 24

what is round robin?

Answer 24

-processor time is divided equally among all running tasks (time slice)
-if process hasn’t finished when time slice is up, it stops running and the computer switches to the next process
-the suspended process will only resume running when it is next allocated processor time
-ensures every task is allocated processor time
-short wait times
-more processes running -> smaller time slice for each process, more urgent tasks are less likely to be completed quickly

Question 25

what are benefits of round robin?

Answer 25

-ensures every task is allocated processor time
-short wait times

Question 26

what are drawbacks of round robin?

Answer 26

-more processes running means smaller time slice for each process
-more urgent tasks are less likely to be completed quickly

Question 27

what is shortest remaining time?

Answer 27

-the processes with the shortest time to completion go first
-until a different task is added with a shorter time to completion, which then takes priority
(different to the shortest job first as a long job which is mostly complete may have a very short time remaining, so it would prioritised)

Question 28

what are benefits of shortest time remaining?

Answer 28

-reduced processing time

Question 29

what are drawbacks of shortest time remaining?

Answer 29

-starvation

Question 30

what is shortest job first?

Answer 30

the shortest instructions are completed first
-reduces wait times
-however can lead to starvation because longer instructions may not get completed

Question 31

what are multilevel feedback queues?

Answer 31

-uses queues of tasks, where each queue has a different level of priority
-processes are chosen from different queues based on priority and how much processing time each process has already had
-initially, processes are added to a queue with a certain level of priority
-if a process uses too much CPU time, it is moved to a lower priority queue
-if a process has been idle for a long time, it is moved to a higher priority queue
-processes that depend on input/output devices require a lot of processing time, so they are kept in high priority queues
-processes that are quick to complete are served first

Question 32

what are benefits of multi level feedback queues?

Answer 32

-all tasks are given processor time
(avoids starvation)
-more important jobs are dealt with sooner
-queues become more balanced the more tasks are processed, ensures no single high priority task dominate the processor’s time

Question 33

what are drawbacks of multi level feedback queues?

Answer 33

-the most complex algorithm
-CPU intensive